What is shock loading?
Shock loading is a sudden load upon a system.
Impact load is the force that mass in motion applies to a fixed object.
Our docks weigh 5 to 10 tons on average. Even aluminum docks get heavy when you add boat lifts, boats, steel roof and composite decking. As long as this load is static, very little force is applied to your anchor winches, cables and mounts. If it moves and is stopped by your cable, a tremendous amount of energy is applied to the anchor system. How much energy? Lets work it out...
Here is the formula for the potential energy of a shock load.
shock load = load x [1 + (1 + (2 x FD x A x E)/(load x L))^1/2]
Load will be the weight of your dock.
FD is falling distance.
A is the cross sectional area of the cable, for example, a 3/8" 7x19 GAC is .472 x .375 x .375 = .066375".
E is elasticity of the wire rope. New wire rope has 11,200,000 psi while wire rope that has been used a while has 15,000,000 psi.
L is the length of your cable from the winch to the mount.
Lets assume your dock is 10,000 lbs. that moved 9 inches on a 3/8" cable that is 80 feet long. A small single slip dock with a 65 foot walkway. Plugging this info into the formula yields 15,770 lbs. of force. It is a realistic scenario as our cables always have a little droop in them that lets the dock move about a foot before becoming tight.
Impact force is shock force applied to an object, be it a car hitting a tree, or your dock pulling on an anchor mount.
The formula for impact force is F = mv/2t.
F is impact force.
m is the mass or weight of your dock.
v is the velocity your dock is moving, how far per second.
t is the time taken to stop.
Same info as above. 10,000 lb. dock moving at a quarter mile per hour, which is 4.4 inches per second, that comes to a stop in .5 seconds has an impact force of 88,000 pounds. This too is a realistic scenario. A quarter mph or 4.4 inches per second is just fast enough for you to see it moving from shore. Half a second is the time the force is applied to your system once the cable becomes tight. There will be deformation of the winch drum lock, the winch mount, the thimble at the end of your cable, the anchor bow shackle, the pole mounted in the ground and a slight stretch of the cable that will take a very short moment to occur... about half a second or less.
Now lets add a dampener to the system. A Dock Spring with an adequate compression strength to halt the movement of your dock will take time to do so. I have watched this occur and it takes about 10 seconds for the dock spring on my 10+ ton dock to reach a state of near solid compression and stop the movement of my dock. So, a10,000 lb. dock moving at a quarter mile per hour, which is 4.4 inches per second, that comes to a stop in 10 seconds has an impact force of 4,400 pounds if there is a dampener on the anchor cable.
I found data on the Fulton brand hand winches similar which that most of us use on our docks. The most common size is 3500 lbs. working load rating. It has a minimum breaking rating of 7000 lbs. This probably a conservative number as I have seen them hold up to much more than that. Cable and shackle ratings are provided on the Bulletproofing your dock page on this site. Our hardware is more than up to the task so long as we prevent shock and impact loading from exceeding their limits.
The examples above are a simplification of the stresses our docks see and don't take into account the force of wind or current loads which add to the shock and impact loads. But once you mitigate the shock and impact loads with a spring, all your anchor system has to work against is the forces from mother nature. You give your dock a fighting chance of survival.
I am not an engineer and don't play one on tv. I got help from Microsoft's Copilot in working out these problems. Its almost like cheating. Wish I had something like Copilot when I was in school.
Dock Springs introduce time into the equation.
They spread the load over time which reduces impact loads to manageable levels.